He will work for CEO Tim Allan, his former deputy in Tony Blair’s office, and will concentrate on strategic corporate comms, crisis management work and mentoring the Portland team.

Campbell told PRWeek: ‘In this economic climate, and despite all the negative definitions of spin, strategic comms is really important. It’s something that Britain, as an industry, does well. So many organisations are tactical when what they need is strategy. I have an understanding of strategy that is pretty profound.’

At Portland, Campbell joins other former Downing Street staffers such as partners Martin Sheehan and Mark Flanagan. The former director of comms in Downing Street has declined to work a full five-day week, because he wishes to pursue his many other interests. These include writing novels, broadcasting, raising money for charities, helping the Labour Party and after-dinner speaking.

‘I’ve considered joining Portland for many years, and now I’ve reached a point where I feel I can take on a new challenge in balance with my various activities,’ said Campbell.

‘Alastair is unique,’ said Allan. ‘He has a huge strategic brain and an extraordinary ability to make things happen. He brings our clients truly world class strategic advice and is an invaluable coach and mentor to our team.’

Campbell’s final instalment of his diaries while working for Tony Blair is due to be published shortly.

Campbell’s PR career

Last year, Campbell was involved in a campaign to boost the image of Kosovo.

He helped Gordon Brown during the 2010 general election, having previously aided in Tony Blair’s 2005 re-election campaign.

In 2005, he was an adviser to the British and Irish Lions rugby tour of New Zealand.